The present invention relates to an X-ray pixel detector, and more exactly to a pixel-camera based i g detector for X-ray photons with high efficiency combined with high resolution.
Silicon devices as CCDs and CMOS pixel detectors are frequently used for X-ray imaging. Due to the low stopping for X-rays in silicon, the detector is generally coated with a scintillating layer. When using scintillating layers for imaging there is a trade-off between quantum efficiency and resolution. In order to get high quantum efficiency for X-rays the layer should be made thick, but that will reduce the spatial resolution in the image. The quantum efficiency for X-rays is one of the most critical parameters for medical X-ray imaging devices since the signal to noise ratio in the image depends on the number of X-ray photons contributing to the image. Since photoelectric absorption is a single event an X-ray photon will either be fully absorbed or pass unnoticed through the detector.
X-ray generators for dental X-ray imaging operate with an accelerating voltage of 60-90 kV giving mean photon energy in the range 30-40 keV. The material thickness required to stop 80% of the X-ray photons is in the range 150-500 xcexcm for the commonly used scintillators. The primary interaction between the photon and the material, photoelectric absorption, is a single event. The light in the scintillator is then generated by a large number of secondary reactions taking place within a few microns from the location of the primary interaction. As a result a flash of light is generated close to the spot of the primary interaction and radiated in all directions. The quantum efficiency for X-rays is then related to the probability for the primary interaction to occur and to a very small extent to the secondary interactions. In the energy range of interest for such an application and with the materials used as scintillators the primary interaction is generally a photoelectric absorption. Compton scattering and other events are less likely to occur.
The light generated in the scintillator is projected onto the sensor with a spot size, which is proportional to the distance between the point of interaction and the position of absorption in the sensor. The projection is also affected by the refractive indexes of the materials the beam will pass. For a typical combination of scintillator and CCD, the scintillator thickness should be less than 100 xcexcm to achieve a spatial resolution greater than 10 line-pairs/mm, as required for dental X-ray imaging.
A method to improve the spatial resolution of thick scintillating layers is to define pixels in the scintillator, as proposed in EP-A2-0 534 683, U.S. Pat. No. 5,059,800 and U.S. Pat. No. 5,831,269 and to make that the light generated within one pixel is confined within that pixel. Pixel definition in scintillators can be done in a number of ways, e.g. columnar growth of scintillator crystal or groove etching in scintillating films. In EP-A2-0 534 683 dicing or cutting is suggested for separating scintillator elements from a large scintillator block, as appropriate for larger lateral dimensions.
The method for columnar growth of scintillating crystals is well known. It has been used to grow CsI for many years. The document WO93/03496 discloses for instance growth of separate columns in different scintillators whereas in U.S. Pat. No. 4,663,187 a scintillator is held close to the melting point resulting in the formation of domains. The disadvantage of techniques for growth of separated columns is that the columns tend to grow together for thick layers and that light will leak to adjacent columns. It is difficult to apply a light reflector between the columns.
Etching of grooves in scintillating materials is considered to be extremely difficult due to the high aspect ratios required by the application. With a pixel size of 50 xcexcm and an allowed area loss of less than 20% the groove width should be less than 5 xcexcm. If the film thickness is 200 xcexcm the aspect ratio will be 40. This aspect ratio can only be realised by advanced silicon processing techniques whereas etching techniques for scintillating materials are far less developed. Nevertheless, U.S. Pat. No. 5,519,227 claims that laser-based micro-machining techniques could be used to define narrow grooves in a scintillating substrate. However, the technique is inherently slow as the laser needs to be scanned several times in every groove. Furthermore, it is not clear whether re-deposition onto the walls will occur as a result of this laser ablation, which could potentially block a narrow groove.
Summarising, various techniques have been proposed for the fabrication of a scintillator array that would provide light guiding of secondary photons to an underlying imaging detector, These techniques are all restricted in one or several aspects: either too large lateral dimensions (cutting, dicing), problems of forming a well-defined narrow wall (laser ablation), cross talk between adjacent pixels (columnar growth technique) or a lengthy processing time (valid for most of these techniques). Finally, deposition of a reflective layer in the grooves is usually suggested to improve light guiding and reduce cross talk. But, none of these fabrication schemes have proposed a detailed scheme how the reflective layer would be produced. This is not an easy task considering the narrow pore geometry and materials involved.
Therefore there is still a desire to develop a device and it""s associated fabrication method, which should be able to handle thick scintillating material layers but with a maintained resolution which corresponds to the individual pixel size. Furthermore, the fabrication technique should preferably be fast, as for a mass scale production type, and relying as much as possible on existing processes and machinery.
The objective of the present invention is to design and develop a fabrication method for an X-ray pixel detector, i.e. an imaging detector for X-ray photons presenting high efficiency combined with high resolution to obtain a high image quality detector while at the same time minimizing the X-ray dose used. The application is particularly important whenever the X-ray photon absorption distance is much longer than the required pixel size.
It is proposed to take advantage of the mature processing tools of the silicon microelectronics technology where lateral dimensions on a micrometer scale may readily be achieved. Thus, a silicon mold is fabricated by high-aspect ratio etching of a silicon substrate for form an array of pores. This array is subsequently oxidized to provide a low refractive index layer in contact with each individual scintillator block, formed by melting a scintillating material into the pores.
A scintillator device according to the present invention presents a structure based on light guiding of secondarily produced scintillating photons in a pixel detector in conjunction with, for instance, a CCD or a CMOS pixel detector. The structure according to the invention presents a matrix having deep pores created by thin walls presenting a pore spacing appropriate to the image detector in use, and may utilize a reflective layer on the walls of the matrix to increase light guiding down to the image detector chip.